Does anyone know the exact distance between the barrels? The diagram appears to show 3.5' or 3.5 feet from the center line which seems fairly close. 3.5 yards? For every other measurement they included a metric measure except this one.

I'm uninformed about the 2017 rules draft, so don't trust me and wait for a real answer. But I feel pretty confident that the meaning is that each set of barrels is 3.5 feet from the center line of the course. The non-metric, conventional measurement is used above-the-line in the other measurements, and they do use either an apostrophe or "yd" to designate feet and yards respectively.

Seven feet doesn't seem like far to move, but it makes sense because the targets are still 15 yards away and are positioned roughly 20-feet edge-to-edge. They'd need to keep the angles fairly restricted for safety, and to keep the overall dimensions of the course reasonable for most clubs and ranges. If the barrels were 7 yards apart, that would put a pretty extreme angle on the outside target from each position.

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The diagram shows 7 feet between the inside edges of the barrels with the inside edge of each barrel being 3.5 feet from the centerline of the course. The barrels are 15 yards from the targets.

I agree with Skiprr, by keeping the barrels close together with the targets 15 yards away you could easily shoot this at an indoor range without any fear of hitting a wall. It should also make it easier for clubs/ranges to setup the Classifier if they have a shooting bay that is not that wide.

[Edited to add a comment about indoor ranges and narrow shooting bays.]

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